A housing for a downhole tool, having a fluid vent assembly comprising a port passing through the wall of the housing to transmit fluid through the port when the port is open, a sealing device to seal the port, and a cutting device to disrupt the sealing device and permit transmission of fluid through the port. The cutting device is driven axially through the sealing device without relative rotation between the two. The cutting device has an asymmetric blade to cut a coupon from the sealing device by a gradual and progressive disruption of the closure, typically cutting through only a portion the closure device, so that the coupon cut from the closure device remains attached to the closure device when the closure device is ruptured to open the seal. Typically the downhole tool comprises a downhole pump, and the housing comprises the shroud of the pump.